With advancement in storage technology, the amount of data that can be stored in storage subsystems, which include hard disk drives, disk array systems, and so forth, has increased dramatically. However, one of the issues associated with storage subsystems is the possibility of failure, which may result in loss of data. To address the risk of data loss, various types of data recovery techniques have been implemented. One type of recovery technique uses tape backup, in which data on a disk storage subsystem is backed up to a tape on a periodic basis. Should the disk storage subsystem fail, then data can be recovered from the tape.
A fault tolerant storage subsystem, such as a RAID (Redundant Array of Inexpensive Disks) subsystem, can also be used to avoid data loss. There are several RAID levels, with RAID level 1 using mirroring to provide fault tolerance, and other RAID levels (e.g., RAID level 4 or 5) using parity to enable the recovery of data.
Another data recovery technique involves the taking of “snapshots” of data, with a snapshot being a copy of data in a “storage unit” taken at a particular time. A “storage unit” refers to a logical unit of data (e.g., a volume) in a storage subsystem. A snapshot of data is also referred to as a point-in-time representation of data. If recovery of data is desired, the storage unit can be restored to a prior state by copying data from a snapshot back into the storage unit.
Multiple snapshots of data stored in a storage unit can be taken at different times. In conventional storage subsystems, once data in a snapshot is restored back to the storage unit, any newer snapshots (that is, snapshots that are more recent than the snapshot used to restore data) are invalidated or disabled because data in such snapshots are no longer coherent or valid. The invalidated or disabled snapshots can no longer be used for subsequent recovery operations, which reduces flexibility in recovering from snapshots. For example, once a user restores data from a given snapshot, the user typically loses the ability to restore data from more recent snapshots.